network connection to agent should be well secured by default

re: !87 (comment 116633)

design doc: https://gitlab.isc.org/isc-projects/stork/-/wikis/Designs/Agent-Server-Security

There are a couple of aspects to be addressed here:

• confidentiality
• mutual authentication
• authorization

This work has been split to separate issues:

• #479 (closed) (keys and certs generation)
• #481 (closed) (agent registration in the server)
• #483 (closed) (agent deployment and authorization UI)
• #485 (closed) (demo update)
• #486 (closed) (docs)
• #507 (closed) (system tests)